This invention relates to flue gas treatment devices, and more particularly, to a venturi-type rod scrubber for effecting the removal of particulate matter from flue gas in a sufficiently effective and efficient manner as to insure that the flue gas, which is being released, after treatment in the subject scrubber, to the atmosphere complies with the Environmental Protection Agency's proposed standards for particulate emissions.
Flue gas treatment devices are not unknown in the prior art. However, most of the effort, which has been directed towards the development of such devices, is of relatively recent vintage. More specifically, until the relatively recent adoption by federal, state, and local governments of standards regulating the amount of contaminants that can be released to the atmosphere, relatively little attention had been focused on the development of devices for treating flue gases to effect the removal therefrom of contaminants. Rather, generally speaking, the practice, which had enjoyed relatively widespread acceptance heretofore, had been one of simply permitting the flue gas, while still in an untreated state, to pass into the atmosphere. Now, the existence of such air-emission standards is having the effect of forcing the operators of emission producing equipment to either provide their equipment with air pollution control devices in an effort to comply with the standards, or else expose themselves to the possibility of incurring relatively severe penalties for failure to comply with these standards.
The issuance of governmental regulations concerning the extent to which air pollutants can be exhausted to the atmosphere has been motivated principally by the concern that exists regarding the harmful effects such pollutants can have on humans as well as the damage caused thereby to the environment. Two of the constituents commonly found to exist in flue gas, and which to-date have been the subject of attention in an effort to effect their elimination from flue gas are sulfur compounds and particulate matter. One of the reasons why the inclusion of sulfur dioxide in the flue gas is viewed as being undesirable is because a characteristic of sulfur dioxide is that it readily combines with water vapor and oxygen to form sulfuric acid, which is known to be capable of posing a hazard to humans as well as being known to have a damage-inducing effect on the environment.
Particulate matter, on the other hand, is unwanted for both health and aesthetic reasons. Namely, when particulate matter is allowed to be discharged into the atmosphere, it normally tends to fall to the ground producing a thin layer thereof that effectively covers everything with which it comes in contact. Although such a layer of particulate matter normally does not pose a serious threat to human health per se, though it is known to cause serious discomfort to those suffering from respiratory ailments, it is unsightly and does commonly occasion the necessity of effecting the more frequent repainting, cleaning, etc. of the exterior of building structures, etc. that have been covered with the layer of particulate matter.
The prior art contains a multiplicity of teachings of a variety of different types of devices that have been designed to be operative to effect the removal of contaminants from flue gas. One such type of device is that known in the prior art as a marble bed. For a description of the nature of the construction and the mode of operation of a prior art form of marble bed, reference may be had to U.S. Pat. No. 3,687,613--Rickard. As set forth therein, marble bed type scrubbers have been utilized primarily for the purpose of effecting the removal of sulfur compounds from flue gases. One major disadvantage, however, of the marble bed type of scrubber is that it has exhibited a susceptibility to becoming clogged in the course of operation, whereupon it loses its effectiveness as a flue gas treatment device.
Another type of device, which has been employed in the prior art to effect the removal of sulfur compounds from flue gases, is that commonly referred to as a spray tower. As known in the prior art, a spray tower comprises a tower in the form of a section of conduit, i.e. ductwork, through which flue gas is made to flow. During the course of its passage through the spray tower, a countercurrent spraying of the flue gas takes place. The effect of this spraying of the flue gas is to produce a washing of the particle-laden flue gas to free the latter from material particularly in the nature of sulfur compounds, that may be combined with the flue gas. Commonly, the spray is in the form of a slurry which contains an additive that has a known affinity for sulfur oxides.
Although spray towers have proven to be effective devices insofar as concerns the removal of sulfur oxides from the flue gas, there has nevertheless still existed a need for the flue gas to undergo additional treatment in order to effect the removal of particulate matter therefrom. One form of device that has been utilized by the prior art for this purpose is that known as a rod scrubber. As its name implies, a rod scrubber consists of a multiplicity of rods preferably arranged in a plurality of parallel layers, that are suitably disposed so as to extend substantially transverse to the path of flow of the flue gas. Normally, a spray means is cooperatively associated with the rods. The latter spray means is operative to discharge a liquid into the path of flow of the flue gas in the same direction as the flue gas is flowing. Reference may be had to U.S. Pat. No. 3,730,499--Jonakin for a teaching of the nature of the construction and the mode of operation of a prior art form of rod scrubber. In the aforesaid Jonakin patent, the rod scrubber is illustrated being employed in association with a device referred to as a demister. Basically, the function of a demister is to effect the separation from the gas stream of most of the fine water droplets that may remain entrained in the flue gas after the latter leaves the rod scrubber.
Notwithstanding the employment by the prior art of devices such as spray towers to effect the removal of sulfur compounds from the flue gas, rod scrubbers to effect the removal of particulate matter from the flue gas, and demisters to effect the separation of fine water droplets from the gas stream, it has nevertheless been found that the flue gas that is being exhausted from the stack to the atmosphere, following the passage thereof through such devices, still retains therein a measurable amount of contaminants. Furthermore, the pursuit of a reduction in the amount of contaminants still being released to the atmosphere with the flue gas has heretofore largely been deferred on the basis of simply representing a goal to be achieved at some future date. However, a change in attitude has been forthcoming in this regard more recently. A major impetus behind this attitudinal change has been the proposal by the Environmental Protection Agency of more stringent particulate emission requirements. Namely, although prior art flue gas treatment devices may have been effective in the past to meet the then prevailing air-emission standards, it is feared that such devices will no longer be adequate to effect the type of treatment of the flue gas that is required in order to comply with the new, and more restrictive standards that are being adopted at all levels of government.
Turning to a consideration solely of that of the subject of particulate removal, and more specifically the matter of the effectiveness of rod scrubbers in accomplishing the function of particulate removal, there are at least two major factors that merit attention as affecting the performance of any specific type of rod scrubber. These are the rate at which the liquid is sprayed into the gas stream, and the extent to which even distribution of the liquid in the gas stream is achieved. These two factors are important because they both affect the degree to which adequate liquid coverage is provided. In turn, the effectiveness and the efficiency with which a rod scrubber performs its function of particulate removal is predicated to a large extent on the degree to which adequate liquid coverage is present. Namely, for effective and efficient particulate removal, it is important that any dust particles that are entrained in the flue gas do become wetted and collected as the flue gas flows through the rod scrubber. That is, to achieve the desired particulate removal by the rod scrubber, it is essential that intimate contact be provided between the liquid and the entrained particles in order to effect the desired agglomeration of the particles and the liquid droplets concomitant with the engagement thereof with the surface of the rods as the flue gas traverses that portion of the rod scrubber, which is of reduced area and wherein the rods are to be found suitably mounted.
Reference may be had to U.S. Pat. No. 4,012,469--Accortt for a teaching of still another prior art form of rod scrubber. In accordance with the teachings of the aforesaid Accortt patent, at least some of the rods of the upper layer thereof in the rod scrubber are provided with openings through which a spraying of liquid countercurrent to the path of flow of the flue gas through the rod scrubber is effected. More specifically, the upper layer of rods is operative as a distribution conduit for the liquid that is introduced into the gas stream. However, although the rod scrubber, which comprises the subject matter of the aforesaid Accortt patent, provides for a complete distribution of liquid in a transverse direction across the throat portion of the rod scrubber, this type of rod scrubber nevertheless suffers from several disadvantages.
For example, in order to insure the existence of an adequate flow of liquid from each of the openings provided in the rods, it is necessary to impose a limitation on the size of the openings so that an adequate flow pressure of the liquid to each of the rod openings can be maintained. If the size of the rod openings were to be made relatively large, this would necessitate a concomitant increase in flow pressure to insure that an adequate rate of spray of liquid is being provided from each rod opening. Otherwise, the desired distribution of the liquid by the distribution conduit established through the provision of the openings in the upper layer of rods would not be achieved.
Secondly, by virtue of their relatively small size and the fact that they face in the direction of flow of the flue gas through the rod scrubber, the rod openings are susceptible to becoming clogged by agglomerations of dust particles and liquid. Namely, as some of these agglomerates traverse the throat region of the rod scrubber, they form deposits on the rods, and in doing so can occasion the blockage of the openings in the rods.
Thus, a need has been evidenced in the prior art for a flue gas treatment device having the capability to effectively and efficiently accomplish the removal from flue gas of the particulates entrained therein. Moreover, a need has been shown to exist to provide such a flue gas treatment device capable of removing a sufficient quantity of the particulates entrained in the flue gas such that compliance with the stringent particulate emission requirements of the Environmental Protection Agency is achieved. Finally, in the interest of accomplishing more effective and efficient particulate removal, a need has been demonstrated for a flue gas treatment device capable of providing improved liquid coverage to the contact surfaces engaged by the particulates in traversing the flue gas treatment device.
It is therefore an object of the present invention to provide a novel and improved flue gas treatment device operative for effecting the removal from flue gas of particulates entrained therein.
It is another object of the present invention to provide such a flue gas treatment device in the form of a venturi-type rod scrubber.
It is still another object of the present invention to provide such a venturi-type rod scrubber, which is characterized in its capability to insure that a proper volume of liquid is sprayed into the gas stream to effect the removal from the flue gas of particulates entrained therein in an effective and efficient manner.
A further object of the present invention is to provide such a venturi-type rod scrubber, which is characterized in its capability to provide an even distribution of liquid across the gas stream.
A still further object of the present invention is to provide such a venturi-type rod scrubber, which is characterized in its capability to provide adequate liquid coverage of the rod contact surfaces such that sufficient particulate removal is effected that compliance with the stringent particulate emission requirements of the Environmental Protection Agency is achieved.
Yet another object of the present invention is to provide such a venturi-type rod scrubber wherein the provision of the desired liquid coverage is accomplished through the utilization of mutually cooperative external and internal spray means.
Yet still another object of the present invention is to provide such a venturi-type rod scrubber that is readily compatible for use with existing power generation units from whence the particle-laden flue gas to be treated in the subject rod scrubber is derived.